G4eeToTwoGammaModel.cc

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// $Id: G4eeToTwoGammaModel.cc 109177 2018-04-03 06:55:14Z gcosmo $
//
// -------------------------------------------------------------------
//
// GEANT4 Class file
//
//
// File name:   G4eeToTwoGammaModel
//
// Author:        Vladimir Ivanchenko on base of Michel Maire code
//
// Creation date: 02.08.2004
//
// Modifications:
// 08-04-05 Major optimisation of internal interfaces (V.Ivanchenko)
// 18-04-05 Compute CrossSectionPerVolume (V.Ivanchenko)
// 06-02-06 ComputeCrossSectionPerElectron, ComputeCrossSectionPerAtom (mma)
// 29-06-06 Fix problem for zero energy incident positron (V.Ivanchenko) 
// 20-10-06 Add theGamma as a member (V.Ivanchenko)
//
//
// Class Description:
//
// Implementation of e+ annihilation into 2 gamma
//
// The secondaries Gamma energies are sampled using the Heitler cross section.
//
// A modified version of the random number techniques of Butcher & Messel
// is used (Nuc Phys 20(1960),15).
//
// GEANT4 internal units.
//
// Note 1: The initial electron is assumed free and at rest.
//
// Note 2: The annihilation processes producing one or more than two photons are
//         ignored, as negligible compared to the two photons process.



//
// -------------------------------------------------------------------
//
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#include "G4eeToTwoGammaModel.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4TrackStatus.hh"
#include "G4Electron.hh"
#include "G4Positron.hh"
#include "G4Gamma.hh"
#include "Randomize.hh"
#include "G4ParticleChangeForGamma.hh"
#include "G4Log.hh"
#include "G4Exp.hh"

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using namespace std;

G4eeToTwoGammaModel::G4eeToTwoGammaModel(const G4ParticleDefinition*,
                                         const G4String& nam)
  : G4VEmModel(nam),
    pi_rcl2(pi*classic_electr_radius*classic_electr_radius)
{
  theGamma = G4Gamma::Gamma();
  fParticleChange = nullptr;
}

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G4eeToTwoGammaModel::~G4eeToTwoGammaModel()
{}

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void G4eeToTwoGammaModel::Initialise(const G4ParticleDefinition*,
                                     const G4DataVector&)
{
  if(fParticleChange) { return; }
  fParticleChange = GetParticleChangeForGamma();
}

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G4double 
G4eeToTwoGammaModel::ComputeCrossSectionPerElectron(G4double kineticEnergy)
{
  // Calculates the cross section per electron of annihilation into two photons
  // from the Heilter formula.

  G4double ekin  = std::max(eV,kineticEnergy);   

  G4double tau   = ekin/electron_mass_c2;
  G4double gam   = tau + 1.0;
  G4double gamma2= gam*gam;
  G4double bg2   = tau * (tau+2.0);
  G4double bg    = sqrt(bg2);

  G4double cross = pi_rcl2*((gamma2+4*gam+1.)*G4Log(gam+bg) - (gam+3.)*bg)
                 / (bg2*(gam+1.));
  return cross;  
}

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G4double G4eeToTwoGammaModel::ComputeCrossSectionPerAtom(
                                    const G4ParticleDefinition*,
                                    G4double kineticEnergy, G4double Z,
            G4double, G4double, G4double)
{
  // Calculates the cross section per atom of annihilation into two photons
  
  G4double cross = Z*ComputeCrossSectionPerElectron(kineticEnergy);
  return cross;  
}

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G4double G4eeToTwoGammaModel::CrossSectionPerVolume(
          const G4Material* material,
          const G4ParticleDefinition*,
                G4double kineticEnergy,
                G4double, G4double)
{
  // Calculates the cross section per volume of annihilation into two photons
  
  G4double eDensity = material->GetElectronDensity();
  G4double cross = eDensity*ComputeCrossSectionPerElectron(kineticEnergy);
  return cross;
}

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// Polarisation of gamma according to M.H.L.Pryce and J.C.Ward, 
// Nature 4065 (1947) 435.

void G4eeToTwoGammaModel::SampleSecondaries(vector<G4DynamicParticle*>* vdp,
              const G4MaterialCutsCouple*,
              const G4DynamicParticle* dp,
              G4double,
              G4double)
{
  G4double posiKinEnergy = dp->GetKineticEnergy();
  G4DynamicParticle *aGamma1, *aGamma2;

  CLHEP::HepRandomEngine* rndmEngine = G4Random::getTheEngine();
   
  // Case at rest
  if(posiKinEnergy == 0.0) {
    G4double cost = 2.*rndmEngine->flat()-1.;
    G4double sint = sqrt((1. - cost)*(1. + cost));
    G4double phi  = twopi * rndmEngine->flat();
    G4ThreeVector dir(sint*cos(phi), sint*sin(phi), cost);
    phi = twopi * rndmEngine->flat();
    G4double cosphi = cos(phi);
    G4double sinphi = sin(phi);
    G4ThreeVector pol(cosphi, sinphi, 0.0);
    pol.rotateUz(dir);
    aGamma1 = new G4DynamicParticle(theGamma, dir, electron_mass_c2);
    aGamma1->SetPolarization(pol.x(),pol.y(),pol.z());
    aGamma2 = new G4DynamicParticle(theGamma,-dir, electron_mass_c2);
    pol.set(-sinphi, cosphi, 0.0);
    pol.rotateUz(dir);
    aGamma2->SetPolarization(pol.x(),pol.y(),pol.z());
    /*
    G4cout << "Annihilation at rest fly: e0= "  << " dir= " <<  dir 
     << G4endl;
    */
  } else {

    G4ThreeVector posiDirection = dp->GetMomentumDirection();

    G4double tau     = posiKinEnergy/electron_mass_c2;
    G4double gam     = tau + 1.0;
    G4double tau2    = tau + 2.0;
    G4double sqgrate = sqrt(tau/tau2)*0.5;
    G4double sqg2m1  = sqrt(tau*tau2);

    // limits of the energy sampling
    G4double epsilmin = 0.5 - sqgrate;
    G4double epsilmax = 0.5 + sqgrate;
    G4double epsilqot = epsilmax/epsilmin;

    //
    // sample the energy rate of the created gammas
    //
    G4double epsil, greject;

    do {
      epsil = epsilmin*G4Exp(G4Log(epsilqot)*rndmEngine->flat());
      greject = 1. - epsil + (2.*gam*epsil-1.)/(epsil*tau2*tau2);
      // Loop checking, 03-Aug-2015, Vladimir Ivanchenko
    } while( greject < rndmEngine->flat());

    //
    // scattered Gamma angles. ( Z - axis along the parent positron)
    //

    G4double cost = (epsil*tau2-1.)/(epsil*sqg2m1);
    if(std::abs(cost) > 1.0) {
      G4cout << "### G4eeToTwoGammaModel WARNING cost= " << cost
       << " positron Ekin(MeV)= " << posiKinEnergy
       << " gamma epsil= " << epsil
       << G4endl;
      if(cost > 1.0) cost = 1.0;
      else cost = -1.0; 
    }
    G4double sint = sqrt((1.+cost)*(1.-cost));
    G4double phi  = twopi * rndmEngine->flat();

    //
    // kinematic of the created pair
    //

    G4double totalEnergy = posiKinEnergy + 2.0*electron_mass_c2;
    G4double phot1Energy = epsil*totalEnergy;

    G4ThreeVector phot1Direction(sint*cos(phi), sint*sin(phi), cost);
    phot1Direction.rotateUz(posiDirection);
    aGamma1 = new G4DynamicParticle (theGamma,phot1Direction, phot1Energy);
    phi = twopi * rndmEngine->flat();
    G4double cosphi = cos(phi);
    G4double sinphi = sin(phi);
    G4ThreeVector pol(cosphi, sinphi, 0.0);
    pol.rotateUz(phot1Direction);
    aGamma1->SetPolarization(pol.x(),pol.y(),pol.z());

    G4double phot2Energy =(1.-epsil)*totalEnergy;
    G4double posiP= sqrt(posiKinEnergy*(posiKinEnergy+2.*electron_mass_c2));
    G4ThreeVector dir = posiDirection*posiP - phot1Direction*phot1Energy;
    G4ThreeVector phot2Direction = dir.unit();

    // create G4DynamicParticle object for the particle2
    aGamma2 = new G4DynamicParticle (theGamma, phot2Direction, phot2Energy);

    pol.set(-sinphi, cosphi, 0.0);
    pol.rotateUz(phot1Direction);
    cost = pol*phot2Direction;
    pol -= cost*phot2Direction;
    pol = pol.unit();
    aGamma2->SetPolarization(pol.x(),pol.y(),pol.z());
    /*
    G4cout << "Annihilation on fly: e0= " << posiKinEnergy
     << " m= " << electron_mass_c2
     << " e1= " << phot1Energy 
     << " e2= " << phot2Energy << " dir= " <<  dir 
     << " -> " << phot1Direction << " " 
     << phot2Direction << G4endl;
    */
  }  
 
  vdp->push_back(aGamma1);
  vdp->push_back(aGamma2);

  // kill primary positron
  fParticleChange->SetProposedKineticEnergy(0.0);
  fParticleChange->ProposeTrackStatus(fStopAndKill);
}

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